In order to sort out the respective roles of the type I and type II insulin-like growth factor (IGF) receptors in mediating biologic responses, we developed a blocking antibody to the type II receptor. We immunized rabbits with type II receptor purified to homogeneity from rat chrondosarcoma chondrocytes. The resulting antiserum blocks the binding of radiolabeled IGF-II to the type II receptor of a variety of cells and membrane preparations. The antiserum does not block the binding of insulin to the insulin receptor nor does it block the binding of IGF-I to the type I receptor. Based on correlations of dose response curves for IGF-II binding and IGF-II-stimulated biologic responses, others had proposed that the type II receptor was important in mediating IGF-II-stimulated growth responses in L6 myoblasts. Accordingly, we examined the ability of IgG prepared from the antireceptor antiserum to mimic or block IGF-II-stimulated biologic responses in L6 myoblasts. We find that concentrations of anti-type II receptor IgG that are sufficient to completely block IGF-II binding do not mimic or block IGF-II-stimulated glucose or amino acid uptake or thymidine incorporation into DNA. The antireceptor IgG does block the degradation of IGF-II in the medium, presumably by blocking receptor-mediated internalization. In the circulation, IGFs are associated with "binding" or "carrier" proteins. There are two size classes of carrier proteins, a Mr=40,000 protein which is the only carrier protein in fetal blood and a Mr=150,000 protein which is under growth hormone control and is found in postnatal blood. There are conflicting data regarding the structure of the 150,000 binding protein. To resolve these conflicting models, we have chemically crosslinked radiolabeled IGF-I and IGF-II to preparations of the 150,000 binding protein and analyzed the radioligand-binding protein complexes by sodium dodecyl sulfate gel electrophoresis and autoradiography. Our data suggest that the 48,000 species generated from the 150,000 binding protein is not simply a dimer of a 25,000 species, contrary to the prediction of the six subunit model of the 150,000 binding protein.